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The formally named SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt flow that is preserved in the desert climate of northern Arizona, USA. The flow is independently dated with an 40Ar/39Ar age of 72 ± 4 ka (2σ) and has undergone negligible erosion and/or burial, making its surface an ideal site for direct calibration of cosmogenic nuclide production rates. Production rates for cosmogenic 26Al have been determined from SP flow quartz in this study and are combined with production rates for 10Be, 14C, and 21Ne (Fenton et al., 2019) to yield a suite of production rate ratios. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rate of 26Al is 25.8 ± 2.5 at/g/yr (2 σ x ‾ ; standard error) using time-independent Lal (1991)/Stone (2000) (St) scaling factors. The St scaled spallogenic 26Al rate is 25.0 ± 2.4 at/g/yr integrated over the past 72 ka. This rate overlaps within 2σ uncertainty with other St-scaled production rates in the literature. SLHL spallogenic 26Al production rates in SPICE quartz (SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations) are nominally lower if time- dependent Sf, Sa, and Lm scaling factors are used, yielding values of 22.9 ± 2.2 at/g/yr, 22.6 ± 2.2 at/g/yr, and 24.1 ± 2.2 at/g/yr (2 σ x ‾ ), respectively. All 26Al production rates in SP flow quartz overlap within 2σ uncertainty, regardless of time independent or time dependent scaling. Production rate ratios for cosmogenic 26Al/10Be, 26Al/14C, and 26Al/21Ne are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2 σ x ‾ ; standard error) of 6.7 ± 0.6, 2.23 ± 0.20, and 1.51 ± 0.13, respectively. This study suggests that, similar to cosmogenic 21Ne and 10Be production rates in SP flow quartz, production rates of cosmogenic 26Al in quartz do not significantly increase when integrated over 72 ka, a time span which includes the period of decreased magnetic strength from 20 to 50 ka.

The SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt with an 40Ar/39Ar age of 72 ± 4 ka (2σ). The flow is preserved in the desert climate of northern Arizona, USA. Its unweathered appearance and the lack of soil development indicate it has undergone negligible erosion and/or burial, making it an ideal site for direct calibration of cosmogenic nuclide production rates. Cross-calibrated production rates and production rate ratios for cosmogenic 21Ne, 10Be, and 14C have been determined from SP flow quartz. Production rate ratios for 21Ne/10Be, 21Ne/14C, and 14C/10Be are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2σ uncertainty) of 4.44 ± 0.32, 1.43 ± 0.10, and 2.85 ± 0.21, respectively. Error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 21Ne, 10Be, and 14C are 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr (2σ), respectively, using time-independent Lal (1991)/Stone (2000) (St) scaling factors. St scaled spallogenic 10Be and 14C rates are 3.73 ± 0.26 and 9.2 ± 0.6 at/g/yr, respectively. 21Ne and 10Be production rates are integrated over the past 72 ka, whereas 14C production rates are integrated over 25 ka, the time at which SP flow quartz has reached saturation with respect to 14C. These rates overlap within 2σ uncertainty with other St-scaled production rates in the literature, including the total reference SLHL 21Ne production rate of Niedermann (2000), which is revised in this paper to 16.8 ± 3.3 at/g/yr (2σ; St scaling) to reflect a recent change in age control at the Sierra Nevada sites. All SLHL production rates are lower if time- dependent Sf, Sa, and Lm scaling factors are used. For example, error-weighted mean, sea-level, high latitude (SLHL) total reference production rates for 10Be as calculated in the CREp online calculator range from 3.49 ± 0.23 to 3.74 ± 0.25 at/g/yr (2σ), using time-dependent Lm scaling factors. Commonly used SLHL 10Be and 14C production rates in the literature were calibrated on surfaces that have been exposed to cosmic rays for less than 20 ka. Between 20 and 50 ka, the geomagnetic field is proposed to have been weaker than it is today. Production rates of cosmogenic nuclides increase during periods of weaker geomagnetic field strength. However, our study finds no measureable difference between St-scaled production rates of cosmogenic 21Ne and 10Be over the past 20 ka and St-scaled 21Ne and 10Be production rates over the past 72 ka. As such, the study suggests that 21Ne and 10Be production rates in quartz were not significantly greater during the proposed period of decreased magnetic strength from 20 to 50 ka.

This study reports an inter-laboratory comparison of the 3He and 4He concentrations measured in the pyroxene material CRONUS-P. This forms part of the CRONUS-Earth and CRONUS-EU programs, which also produced a series of natural reference materials for in situ produced ^(26)Al, ^(10)Be, ^(14)C, ^(21)Ne and ^(36)Cl. Six laboratories (GFZ Potsdam, Caltech Pasadena, CRPG Nancy, SUERC Glasgow, BGC Berkeley, Lamont New York) participated in this intercomparison experiment, analyzing between 5 and 22 aliquots each. Intra-laboratory results yield ^3He concentrations that are consistent with the reported analytical uncertainties, which suggests that ^3He is homogeneous within CRONUS-P. The inter-laboratory dataset (66 determinations from the 6 different labs) is characterized by a global weighted mean of (5.02 ± 0.12) × 10^9 at g^(−1) with an overdispersion of 5.6% (2σ). ^4He is characterized by a larger variability than ^3He, and by an inter-lab global weighted mean of (3.60 ± 0.18) × 10^(13) at g^(−1) (2σ) with an overdispersion of 10.4% (2σ). There are, however, some systematic differences between the six laboratories. More precisely, 2 laboratories obtained mean ^3He concentrations that are about 6% higher than the clustered other 4 laboratories. This systematic bias is larger than the analytical uncertainty and not related to the CRONUS-P material (see Schaefer et al., 2015). Reasons for these inter-laboratory offsets are difficult to identify but are discussed below. To improve the precision of cosmogenic ^3He dating, we suggest that future studies presenting cosmogenic 3He results also report the ^3He concentration measured in the CRONUS-P material in the lab(s) used in a given study.

Time-averaged fault slip-rates can be established by reliably dating the abandonment of an alluvial deposit that has been displaced by Quaternary movement along a cross-cutting fault. Unfortunately, many Quaternary dating techniques are hindered by uncertainties inherent to individual geochronometers. Such uncertainties can be minimised by combining multiple independent techniques. In this study, we combine 10Be exposure dating of boulder tops and U-series dating of layered pedogenic carbonate cements accumulated on the underside of clasts from two separate alluvial surfaces. These surfaces are both displaced by the active Ölgiy strike-slip fault in the Mongolian Altay Mountains. We date individual layers of pedogenic carbonate, and for the first time apply a Bayesian statistical analysis to the results to develop a history of carbonate accumulation. Our approach to the U-series dating provides an age of initiation of carbonate cement formation and avoids the problem of averaging contributions from younger layers within the carbonate. The U-series ages make it possible to distinguish 10Be samples that have anomalously young exposure ages and have hence been subject to the effects of post-depositional erosion or exhumation. The combination of 10Be and U-series dating methods provides better constrained age estimates than using either method in isolation and allows us to bracket the abandonment ages of the two surfaces as 18.0-28.1kyr and 38.4-76.4kyr. Our ages, combined with measurements of the displacement of the surfaces, yield a right-lateral slip-rate for the Ölgiy fault of 0.3-1.3mmyr-1, showing that it is a relatively important structure within the active tectonics of Mongolia and that it constitutes a substantial hazard to local populations.

K–Ar ages of young basalts ( 40 Ar and have concluded that K–Ar ages of young basalts in these fields tend to be inaccurate. This new study in the San Francisco volcanic field presents 3 He c and 21 Ne c ages yielded by olivine and pyroxene collected from three Pleistocene basalt flows – the South Sheba (∼190 ka), SP (∼70 ka), and Doney Mountain (∼67 ka) lava flows, – and from one Holocene basalt, the Bonito Lava Flow (∼1.4 ka) at Sunset Crater. These data indicate that, in two of three cases, 40 Ar/ 39 Ar and K–Ar ages of the young basalts agree well with cosmic-ray surface exposure ages of the same lava flow, thus suggesting that excess 40 Ar is not always a problem in young basalt flows in the San Francisco volcanic field. The exposure age of the Bonito lava flow agrees within uncertainty with dendrochronological and archeological age determinations. K–Ar and cosmogenic 3 He and 21 Ne ages from the SP flow are in agreement and much older than the OSL age (5.5–6 ka) reported for this lava flow. Furthermore, if the non-cosmogenic ages are assumed to be accurate, the subsequent calculated production rates at South Sheba and SP flow sample sites agree well with values in the literature.

Rivers carved into uplifted plateaus are commonly disrupted by discrete events from the surrounding landscape, such as lava flows or large mass movements. These disruptions are independent of slope, basin area, or channel discharge, and can dominate aspects of valley morphology and channel behavior for many kilometers. We document and assess the effects of one type of disruptive event, lava dams, on river valley morphology and incision rates at a variety of time scales, using examples from the Owyhee River in southeastern Oregon. Six sets of basaltic lava flows entered and dammed the river canyon during two periods in the late Cenozoic ca. 2 Ma–780 ka and 250–70 ka. The dams are strongly asymmetric, with steep, blunt escarpments facing up valley and long, low slopes down valley. None of the dams shows evidence of catastrophic failure; all blocked the river and diverted water over or around the dam crest. The net effect of the dams was therefore to inhibit rather than promote incision. Once incision resumed, most of the intracanyon flows were incised relatively rapidly and therefore did not exert a lasting impact on the river valley profile over time scales >10 6 yr. The net long-term incision rate from the time of the oldest documented lava dam, the Bogus Rim lava dam (≤1.7 Ma), to present was 0.18 mm/yr, but incision rates through or around individual lava dams were up to an order of magnitude greater. At least three lava dams (Bogus Rim, Saddle Butte, and West Crater) show evidence that incision initiated only after the impounded lakes filled completely with sediment and there was gravel transport across the dams. The most recent lava dam, formed by the West Crater lava flow around 70 ka, persisted for at least 25 k.y. before incision began, and the dam was largely removed within another 35 k.y. The time scale over which the lava dams inhibit incision is therefore directly affected by both the volume of lava forming the dam and the time required for sediment to fill the blocked valley. Variations in this primary process of incision through the lava dams could be influenced by additional independent factors such as regional uplift, drainage integration, or climate that affect the relative base level, discharge, and sediment yield within the watershed. By redirecting the river, tributaries, and subsequent lava flows to different parts of the canyon, lava dams create a distinct valley morphology of flat, broad basalt shelves capping steep cliffs of Tertiary sediment. This stratigraphy is conducive to landsliding and extends the effects of intracanyon lava flows on channel geomorphology beyond the lifetime of the dams.

Two rock avalanches in Troms County – the Grotlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 104 and 5.06 × 104 at 10Be/g using the 1.387 Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Holen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grotlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation.

The Namib Sand Sea is one of the world’s oldest and largest sand deserts1, yet little is known about the source of the sand in this, or other large deserts2. In particular, it is unclear whether the sand is derived from local sediment or comes from remote sources. The relatively uniform appearance of dune sands and low compositional variability within dune fields3 make it difficult to address this question. Here we combine cosmogenic-nuclide measurements and geochronological techniques to assess the provenance and migration history of sand grains in the Namib Sand Sea. We use U–Pb geochronology of detrital zircons to show that the primary source of sand is the Orange River at the southern edge of the Namib desert. Our burial ages obtained from measurements of the cosmogenic nuclides 10Be, 26Al and 21Ne suggest that the residence time of sand within the sand sea is at least one million years. We therefore conclude that, despite large climatic changes in the Namib region associated with Quaternary glacial–interglacial cycles4, 5, the area currently occupied by the Namib Sand Sea has never been entirely devoid of sand during the past million years.

We report cosmogenic surface exposure 10Be ages of 21 boulders on moraines in the Jeullesh and Tuco Valleys, Cordillera Blanca, Peru (∼10°S at altitudes above 4200 m). Ages are based on the sea-level at high-latitude reference production rate and scaling system of Lifton et al. (2005. Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications. Earth and Planetary Science Letters 239, 140–161) in the CRONUS-Earth online calculator of Balco et al. (2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174–195). Using the Lifton system, large outer lateral moraines in the Jeullesh Valley have a 10Be exposure age of 12.4 ka, inside of which are smaller moraine systems dated to 10.8, 9.7 and 7.6 ka. Large outer lateral moraines in the Tuco Valley have a 10Be exposure age of 12.5 ka, with inner moraines dated to 11.3 and 10.7 ka. Collectively, these data indicate that glacier recession from the Last Glacial Maximum (LGM) in the Cordillera Blanca was punctuated by three to four stillstands or minor advances during the period 12.5–7.6 ka, spanning the Younger Dryas Chronozone (YDC; ∼12.9–11.6 ka) and the cold event identified in Greenland ice cores and many other parts of the world at 8.2 ka. The inferred fluctuations of tropical glaciers at these times, well after their withdrawal from the LGM, indicate an increase in precipitation or a decrease in temperature in this region. Although palaeoenvironmental records show regional and temporal variability, comparison with proxy records (lacustrine sediments and ice cores) indicate that regionally this was a cold, dry period so we ascribe these glacier advances to reduced atmospheric temperature rather than increased precipitation.

The confidence in surface exposure dating and related research, such as erosion rate studies or burial dating, strongly depends on the accuracy and precision of the currently used production rates of in situ-produced cosmogenic nuclides. Reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes. Here we use surface samples from the 760 ± 2 ka old Bishop Tuff in eastern California to quantify the 10 Be/ 21 Ne and 26 Al/ 21 Ne production rate ratios in quartz. Our determination is based on (1) measured nuclide concentrations of cosmogenic 10 Be, 21 Ne, and 26 Al, (2) a conservative estimate for the erosion of the tuff in the Volcanic Tableland area, which we base on our previously published 21 Ne concentrations [Goethals, M.M., Niedermann, S., Hetzel, R., Fenton, C.R., 2009. Determining the impact of faulting on the rate of erosion in a low-relief landscape: A case study using in situ produced 21 Ne on active normal faults in the Bishop Tuff, California. Geomorphology 103, 401–413] and a conservative estimate for the uncertainty of the 21 Ne production rate, and (3) the assumption of steady-state erosion. Other assumptions, such as the applied scaling procedure, the muon contribution to nuclide production, or the attenuation lengths of neutrons and muons in rock, do not substantially affect the results. Based on 13 samples, the following average production rate ratios and conservative uncertainty estimates are obtained for sea level, high latitude, open sky, and rock surface: 0.249 ± 0.009 or 0.232 ± 0.009 for 10 Be/ 21 Ne using 10 Be half-lives of 1.51 and 1.39 Ma, respectively, and 1.80 ± 0.09 for 26 Al/ 21 Ne (for an 26 Al half-life of 0.72 Ma). The 10 Be/ 21 Ne and the 26 Al/ 21 Ne production ratios are consistent with currently used production rates but the ratios are much more precise than previous determinations. The resulting 26 Al/ 10 Be production ratio of 7.23 ± 0.45 (for a 10 Be half-life of 1.51 Ma) or 7.76 ± 0.49 (for a 10 Be half-life of 1.39 Ma) is high when compared to previously published values. We discuss reasons for this difference, amongst them the possibility that 26 Al analyses in general might be compromised by artefacts affecting the stable Al concentration measurements. When combined with a 10 Be production rate of 5.01 at g − 1 a − 1 for the 1.51 Ma half-life (or 4.61 at g − 1 a − 1 for the 1.39 Ma half-life), our production ratios convert to 21 Ne and 26 Al production rates of 20.1 and 36.2 at g − 1 a − 1 (or 19.9 and 35.7 at g − 1 a − 1 ), respectively.

At least one large, late Pleistocene flood traveled into the Owyhee River as a result of a rise and subsequent outburst from pluvial Lake Alvord in southeastern Oregon. Lake Alvord breached Big Sand Gap in its eastern rim after reaching an elevation of 1292 m, releasing 11.3 km 3 of water into the adjacent Coyote Basin as it eroded the Big Sand Gap outlet channel to an elevation of about 1280 m. The outflow filled and then spilled out of Coyote Basin through two outlets at 1278 m and into Crooked Creek drainage, ultimately flowing into the Owyhee and Snake Rivers. Along Crooked Creek, the resulting flood eroded canyons, stripped bedrock surfaces, and deposited numerous boulder bars containing imbricated clasts up to 4.1 m in diameter, some of which are located over 30 m above the present-day channel. Critical depth calculations at Big Sand Gap show that maximum outflow from a 1292- to 1280-m drop in Lake Alvord was ∼ 10,000 m 3 s − 1 . Flooding became confined to a single channel approximately 40 km downstream of Big Sand Gap, where step-backwater calculations show that a much larger peak discharge of 40,000 m 3 s − 1 is required to match the highest geologic evidence of the flood in this channel. This inconsistency can be explained by (1) a single 10,000 m 3 s − 1 flood that caused at least 13 m of vertical incision in the channel (hence enlarging the channel cross-section); (2) multiple floods of 10,000 m 3 s − 1 or less, each producing some incision of the channel; or (3) an earlier flood of 40,000 m 3 s − 1 creating the highest flood deposits and crossed drainage divides observed along Crooked Creek drainage, followed by a later 10,000 m 3 s − 1 flood associated with the most recent shorelines in Alvord and Coyote Basins. Well-developed shorelines of Lake Alvord at 1280 m and in Coyote Basin at 1278 m suggest that after the initial flood, postflood overflow persisted for an extended period, connecting Alvord and Coyote Basins with the Owyhee River of the Columbia River drainage. Surficial weathering characteristics and planktonic freshwater diatoms in Lake Alvord sediment stratigraphically below Mt. St. Helens set Sg tephra, suggest deep open-basin conditions at ∼ 13–14 ka ( 14 C yr) and that the flood and prominent shorelines date to about this time. But geomorphic and sedimentological evidence also show that Alvord and Coyote Basins held older, higher-elevation lakes that may have released earlier floods down Crooked Creek.

Pleistocene basaltic lava dams and outburst‐flood deposits in the western Grand Canyon, Arizona, have been correlated by means of cosmogenic 3He (3Hec) ages and concentrations of SiO2, Na2O, K2O, and rare earth elements. These data indicate that basalt clasts and vitroclasts in a given outburst‐flood deposit came from a common source, a lava dam. With these data, it is possible to distinguish individual dam‐flood events and improve our understanding of the interrelations of volcanism and river processes. At least five lava dams on the Colorado River failed catastrophically between 100 and 525 ka; subsequent outburst floods emplaced basalt‐rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites. Chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long‐lasting dam complexes. These chemical data, in combination with age constraints, increase our ability to correlate lava dams and outburs...

Hazard mitigation for extreme events such as debris flows requires geologic mapping and chronologic information, particularly for alluvial fans near mountain fronts in the southwestern United States. In July 2006, five consecutive days of monsoonal storms caused hundreds of debris flows in southeastern Arizona, particularly in the southern Santa Catalina Mountains north of Tucson. Before 2006, no historical debris flows from the Santa Catalina Mountains reached the populated mountain front, although abundant evidence of prehistoric debris flows is present on downslope alluvial fans. We used a combination of surficial geologic mapping and 10 Be exposure dating to produce a debris-flow history for Pima and Finger Rock Canyons. The largest debris flows, of latest Pleistocene to early Holocene age, covered much of the apices of alluvial fans formed at the mouths of these canyons and extended up to 3 km downslope. These debris-flow deposits were inset against higher and older alluvial surfaces with few debris-flow deposits of late Pleistocene age. The 10 Be ages in this study have considerable scatter for surfaces believed to be of uniform age, indicating the dual possibilities of inheritance from previous cosmic-ray exposure, as well as the potential for composite deposits derived from numerous debris flows. We then used an empirical inundation model, LAHARZ, to assess probable magnitudes of the older debris flows to evaluate possible initiation mechanisms. In-channel and terrace storage within the canyons is not sufficient to generate volumes likely needed to produce the larger late Pleistocene to early Holocene debris-flow deposits. The abundance of latest Pleistocene and early Holocene deposits suggests that large debris flows were generated during the instability associated with climate and vegetation changes at the Pleistocene–Holocene transition. Under present watershed conditions with limited sediment supplies, modern debris-flow hazards are generally limited to within mountains and near mountain fronts.

Based on cosmogenic 10Be and 26Al analyses in 15 individual detrital quartz pebbles (16e21 mm) and cosmogenic 10Be in amalgamated medium sand (0.25e0.50 mm), all collected from the outlet of the upper Gaub River catchment in Namibia, quartz pebbles yield a substantially lower average denudation rate than those yielded by the amalgamated sand sample. 10Be and 26Al concentrations in the 15 indi- Accepted 9 April 2012 Available online xxx vidual pebbles span nearly two orders of magnitude (0.22 ± 0.01 to 20.74 ± 0.52 x 10 6 10 Be atoms g-1 and 1.35 ± 0.09 to 72.76 ± 2.04 x 106 26Al atoms g-1, respectively) and yield average denudation rates of w0.7 m Myr-1 (10Be) and w0.9 m Myr-1 (26Al). In contrast, the amalgamated sand yields an average Keywords: Beryllium-10 10Be concentration of 0.77 ± 0.03 x 106 atoms g-1, and an associated mean denudation rate of Aluminium-26 Neon-21 Cosmogenic nuclide Grain size bias Namibia 9.6 ± 1.1 m Myr-1, an order of magnitude greater than the rates obtained for the amalgamated pebbles. The inconsistency between the 10Be and 26Al in the pebbles and the 10Be in the amalgamated sand is likely due to the combined effect of differential sediment sourcing and longer sediment transport times for the pebbles compared to the sand-sized grains. The amalgamated sands leaving the catchment are an aggregate of grains originating from all quartz-bearing rocks in all parts of the catchment. Thus, the cosmogenic nuclide inventories of these sands record the overall average lowering rate of the landscape. The pebbles originate from quartz vein outcrops throughout the catchment, and the episodic erosion of the latter means that the pebbles will have higher nuclide inventories than the surrounding bedrock and soil, and therefore also higher than the amalgamated sand grains. The order-of-magnitude grain size bias observed in the Gaub has important implications for using cosmogenic nuclide abundances in deposi- tional surfaces because in arid environments, akin to our study catchment, pebble-sized clasts yield substantially underestimated palaeo-denudation rates. Our results highlight the importance of carefully considering geomorphology and grain size when interpreting cosmogenic nuclide data in depositional surfaces.

Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic 3He (3Hec) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, < 350 ka, < 150 ka, and < 63 ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2 Ma. 3Hec data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in 3Hec ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.

Precise 40Ar/39Ar age determinations made on basalt groundmass collected from the SP and upper and lower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields of Arizona, USA, yield ages of 72 ± 4, 97 ± 10, and 123 ± 12 ka (2σ; relative to Renne et al., 2010, 2011, full external precision), respectively. Previous ages of the SP lava flow include a K–Ar age of 70 ± 8 ka and OSL ages of 5.5–6 ka. 40Ar/39Ar age constraints, relative to the optimization model of Renne et al. (2010, 2011), of 81 ± 50 and 118 ± 64 ka (2σ; full external precision) were previously reported for the upper and lower Bar Ten lava flows, respectively. The new 40Ar/39Ar ages are within uncertainty of previous age constraints, and are more robust, accurate, and precise. Preliminary cosmogenic 3He and 21Ne production rates from the Bar Ten flows reported by Fenton et al. (2009) are updated here, to account for the improved quality of the 40Ar/39Ar data. The new 40Ar/39Ar age for the SP flow yields cosmogenic 3He and 21Ne production rates for pyroxene (119 ± 8 and 26.8 ± 1.9 at/g/yr; error-weighted mean, 2σ uncertainty; Dunai (2000) scaling method) that are consistent with production rate values reported throughout the literature. The 40Ar/39Ar and cosmogenic 3He and 21Ne data support field observations indicating the SP flow has undergone negligible erosion. The SP flow contains co-existing phenocrysts of olivine and pyroxene, as well as xenocrysts of quartz in a fine-grained groundmass facilitating cross-calibration of cosmogenic production rates and production-rate (3He, 10Be, 14C, 21Ne, 26Al, and 36Cl). Thus, we propose the SP flow is an excellent location for a cosmogenic nuclide production-rate calibration site (SPICE: the SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations).

Lundy lies in a strategic geographical position for understanding the glacial history of the British Isles. The island bears evidence of glaciation, largely in the form of ice-moulded bedrock and glacially-transported boulders - an unusual occurrence this far south in the British Isles. Irish Sea ice penetrated the western Bristol Channel overriding Lundy from the northwest during the last phase of glaciation in this area. The results of paired terrestrial cosmogenic nuclide analyses ( 26Al/ 10Be) constrain the timing of this extensive glaciation and provide, for the first time, an age for the exposure of Lundy granite following deglaciation. The results from nine paired samples yield 26Al/ 10Be exposure ages of 31.4-48.8 ka ( 10Be) and 31.7-60.0 ka ( 26Al). This challenges the view that any glaciation this far south must belong to Middle Pleistocene glaciations, such as the Anglian Stage (c. 480-420 ka) and a Devensian age for the last glaciation is consistent with findings from the Isles of Scilly further south. However, the findings suggest early-mid Devensian (marine isotope stage (MIS) 4-3) glaciation of Lundy. It also implies that the island was exposed or covered for a short time by non-erosive cold-based ice at the global Last Glacial Maximum (LGM) during MIS 2 (26-21 ka). The potential exposure of the island throughout MIS 2 contrasts with the evidence from the Isles of Scilly and the Celtic Sea, which were glaciated at the LGM. © 2012 Elsevier Ltd.

This paper presents results of the analysis of paired cosmogenic isotopes (10Be and 26Al) from eight quartz-rich samples collected from ice-moulded bedrock on the Aran ridge, the highest land in the British Isles south of Snowdon. On the Aran ridge, comprising the summits of Aran Fawddwy (905 m a.s.l.) and Aran Benllyn (885 m a.s.l.), 26Al and 10Be ages indicate complete ice coverage and glacial erosion at the global Last Glacial Maximum (LGM). Six samples from the summit ridge above 750–800 m a.s.l. yielded paired 10Be and 26Al ages ranging from 17.2 to 34.4 ka, respectively. Four of these samples are very close in age (10Be ages of 17.5 ± 0.6, 17.5 ± 0.7, 19.7 ± 0.8 and 20.0 ± 0.7 ka) and are interpreted as representing the exposure age of the summit ridge. Two other summit samples are much older (10Be ages of 27.5 ± 1.0 and 33.9 ± 1.2 ka) and these results may indicate nuclide inheritance. The 26Al/10Be ratios for all samples are indistinguishable within one-sigma uncertainty from the production rate ratio line, indicating that there is no evidence for a complex exposure history. These results indicate that the last Welsh Ice Cap was thick enough to completely cover the Aran ridge and achieve glacial erosion at the LGM. However, between c. 20 and 17 ka ridge summits were exposed as nunataks at a time when glacial erosion at lower elevations (below 750–800 m a.s.l.) was achieved by large outlet glaciers in the valleys surrounding the mountains. Copyright © 2011 John Wiley & Sons, Ltd.

The Atlas Mountains display evidence of extensive Pleistocene glaciations. The largest ice fields and valley glaciers formed in the Toubkal massif where multiple glacial advances are recorded. A limited number of preliminary 10 Be analyses from the oldest moraines have yielded exposure ages as old as 76 ka, intermediate moraines have yielded an age of 24 ka, whilst the youngest Pleistocene moraines correlate the Younger Dryas (12.9–11.7 ka).

Normal faults exposed in the Volcanic Tableland in the southeastern part of the Bishop Tuff provide a unique opportunity to investigate whether tectonic faulting and associated fracturing control the rate of erosion in the vicinity of actively growing faults. If fracturing increases with fault displacement, erosion close to the faults should be more rapid for faults with higher displacement. Here we use in situ produced cosmogenic 21 Ne to quantify the erosion rate in the flat footwall blocks of three normal faults that have maximum vertical displacements between ∼ 12 and ∼ 160 m. 21 Ne concentrations in surface samples from volcanic bedrock and desert pavements along the three faults are nearly uniform and demonstrate that erosion is spatially invariant, both along individual faults and between faults with different tectonic offsets. Hence, faulting and fracturing of rocks adjacent to the active normal faults in the Bishop Tuff do not have a significant impact on the rate of erosion in the sub-horizontal footwall blocks next to the faults. This is further supported by samples from sites which are unaffected by faulting but record a similar erosion rate as samples located near faults. Using the density of the Bishop Tuff and a published 40 Ar/ 39 Ar eruption age for the tuff (760 ± 2 ka), the absolute amount of erosion since eruption has been quantified. At most sample sites the time-integrated amount of erosion ranges between ∼ 0.9 and ∼ 1.9 m, which is equivalent to average erosion rates between ∼ 1.2 and ∼ 2.5 m/Ma. Samples of bedrock-derived clasts from desert pavements have slightly higher 21 Ne concentrations than adjacent bedrock outcrops and thus yield higher apparent exposure ages (calculated assuming no erosion). Thus, desert pavements are more suitable for determining the minimum eruption ages of yet-undated welded tuffs than bedrock exposures. Desert pavements from the Bishop Tuff yield exposure ages that are ∼ 60% of the tuff's 40 Ar/ 39 Ar eruption age.

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 109 m3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 105 m3 s−1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 104 m3 s−1) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 104 m3 s−1). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>105 m3 s−1) known worldwide and in the top ten largest floods in North America.

The Grand Falls basalt lava flow in northern Arizona was emplaced in late Pleistocene time. It flowed 10 km from its vent area to the Little Colorado River, where it cascaded into and filled a 65-m-deep canyon to form the Grand Falls lava dam. Lava continued ∼25 km downstream and ∼1 km onto the far rim beyond where the canyon was filled. Subsequent fluvial sedimentation filled the reservoir behind the dam, and eventually the river established a channel along the margin of the lava flow to the site where water falls back into the preeruption canyon. The ca. 150 ka age of the Grand Falls flow provided by whole-rock K-Ar analysis in the 1970s is inconsistent with the preservation of centimeter-scale flow-top features on the surface of the flow and the near absence of physical and chemical weathering on the flow downstream of the falls. The buried Little Colorado River channel and the present-day channel are at nearly the same elevation, indicating that very little, if any, regional downcutting has occurred since emplacement of the flow. Newly applied dating techniques better define the age of the lava dam. Infrared- stimulated luminescence dating of silty mudstone baked by the lava yielded an age of 19.6 ± 1.2 ka. Samples from three noneroded or slightly eroded outcrops at the top of the lava flow yielded 3He cosmogenic ages of 16 ± 1 ka, 17 ± 1 ka, and 20 ± 1 ka. A mean age of 8 ± 19 ka was obtained from averaging four samples using the 40Ar/39Ar step-heating method. Finally, paleomagnetic directions in lava samples from two sites at Grand Falls and one at the vent area are nearly identical and match the curve of magnetic secular variation at ca. 15 ka, 19 ka, 23 ka, and 28 ka. We conclude that the Grand Falls flow was emplaced at ca. 20 ka.

The Toroweap and Hurricane faults, considered to be the most active in Arizona, cross the Uinkaret volcanic field in the western Grand Canyon. These normal faults are downthrown to the west, and the Colorado River crosses these faults as it flows west in the Grand Canyon. Cosmogenic 3 He ( 3 He c ) dates on basalt flows and related landforms are used to calculate vertical displacement rates for these faults. The two faults cross unruptured alluvial fans dated as 3 ka (Toroweap) and 8 ka (Hurricane), and 10 other landforms that range in age from 30 to 400 ka are displaced. Middle and late Quaternary displacement rates of the Toroweap and Hurricane faults are 70–180 and 70–170 m/m.y., respectively. On the basis of these rates, the combined displacement of 580 m on these faults could have occurred in the past 3 to 5 m.y. All 3 He c dates are younger than existing K- Ar dates and are consistent with new 40 Ar/ 39 Ar dates and existing thermoluminescence (TL) dates on basalt flows. These different dating techniques may be combined in an analysis of displacement rates. Downcutting rates for the Colorado River in the eastern Grand Canyon (400 m/m.y.) are at least double the downcutting rates west of the faults (70–160 m/m.y.). Faulting probably increased downcutting in the eastern Grand Canyon relative to downcutting in the western Grand Canyon during the late Quaternary.